1. Field of the Invention
This invention relates to a TTL (Through-The-Lens) type focus detecting device for use in single lens reflex cameras.
2. Description of the Prior Art
It has already been known to provide for optical instruments such as a single lens reflex cameras with so-called TTL type focus detecting devices in various forms. Of these, for example, according to one proposal in Japanese Patent Laid Open Publication No. Sho 52-138924, the objective lens system which is to be focused on an object to be photographed is associated with two lenslets arranged behind a plane equivalent to the focal plane (film plane) in substantially symmetrical parallelism with respect to the optical axis of said objective lens system so that, as a sharp image of the object formed by said lens system changes its position along the optical axis thereof, the positions at which said two lenslets form images are caused to shift relatively in directions substantially perpendicular to the optical axes of the lenslets. By photoelectrically detecting the relative positional relation between the two images formed by the said two lenslets, the focusing condition of the objective lens system to the object to be photographed is assessed. Since this focus detecting device relies not upon a blur of images focused by the objective, that is, the degree of sharpness of the images, but instead upon the extent of relative positional relation between the positions at which the images are formed by the two lenslets, and which is commensurate with the degree of focus of the objective, the various difficult problems which are encountered when the above-described so-called image sharpness sensing type device is put into practice have been eliminated and an advantage of considerably improving the accuracy of focus detection over the so-called image sharpness sensing type device can be expected. In addition thereto, it is also made possible to discriminate with ease whether the out-of-focus condition is due to the far, or near focus.
However, even in such type device, when put it into practice, that is, in order to incorporate it in an instrument, for example, a single lens reflex camera, with the assurance of bringing its functions into full play, there are still many more drawbacks which require improvement. For example, as is understandable from the fact that the above-mentioned conventional proposal applies a simple idea to the positioning of the two lenslets as behind the prescribed focal plane of the objective in parallelism on the opposite side of the optical axis thereof, in parallellism on the opposite side of the optical axis thereof, the use of such focus detecting optical system makes it impossible to limit the light beams incident upon the two lenslets to only those of the beams exiting from the objective which occur at equivalent locations to each other and therefore to always assure what may be called an identity of the two images formed by the respective lenslets. This leads to a drawback wherein the output of the photoelectronic detector is not always sufficiently responsible for the relative change of the focal point of the two images. Another drawback arising from such arrangement of the focus detecting optical system is that the two images formed by the above-described two lenslets are largely affected by a change in the angular field of the objective lens system. This point also contributes to a reduction in the detection accuracy. Still another drawback is that, because said two lenslets tend to mutilate the light beam, or to lower the amount of light, it is made increasingly difficult to maintain the accuracy of focus detection at an acceptable level.
Another aspect of the proposal is that, for the above-described photoelectronic detector, use is made of resistance value change type photoconductive cells, and said photoconductive cells are positioned to receive the images formed by the lenslets, while a change in the difference between the resistance values of the photoconductive cells, that is, balance or unbalance is detected by the bridge circuit. Thus, the positional relationship of the two images, therefore, the focus condition of the objective optical system for the object is detected. However, the use of the only photoconductive cells in such simple way is not sufficient as the detection of a difference in the relative position of the two images is so difficult. Therefore, even with respect to this, a further improvement is desired.
On the other hand, aside from Japanese Patent Laid Open Publication No. Sho 52-138924, according to, for example, Japanese Patent Laid Open Publication No. Sho 51-15432, behind the objective lens system, the image-forming light beam is split off of a portion at each of two positions symmetrical to each other with respect to the optical axis of the system. Then, these portions form images, each on an array of a plurality of photosensitive elements arranged in a plane conjugate to the prescribed focal plane of the objective lens system. Then, the electrical outputs from the two arrays of photosensitive elements are compared with each other to measure the difference in the relative position of the two images formed with the two portions split off the image forming light beam, therefore, the degree of focus of the objective lens system for an object to be photographed.
This or the latter TTL type focus detecting device is, because of its using a plurality of photosensitive elements in sensing each of the two images, superior to the former one in improving the accuracy of focus detection. But, this gives rise to another drawback wherein as the extraction of portions out of the image forming light beam from the objective lens system requires the use of very small reflection mirrors or the like means, it is far more difficult than expected to obtain an image quality good enough to achieve the focus detection. Particularly, the change in the angular field of the objective lens system greatly affects the stability of focus detection. A further problem arises from the loss in the light amount. As a result, it is very difficult to achieve a remarkable improvement of the reliability and accuracy of focus detection.
Although these devices are well suited for adaptation to the focusing of an objective lens of such a single lens reflex camera, the objective lens is generally made interchangeable, and a wide variety of dimensions (namely, focal length, full open F-number, etc.) are available. The camera works with the selective use of a desired interchangeable lens. It is noted here that as the large aperture lenses such as F/1.2, F/1.4, F/1.8 and F/2 are compared with the small aperture lenses such as F/4, F/5.6 and F/8, when to take light beams off the light from the objective lens at two regions lying in symmetry with respect to the optical axis, as has been stated above, while the means for obtaining of such partial light beams is provided in fixed relation to the lens with reference to, for example, a middle aperture value, the use of one of the former or large aperture lenses leads to a relatively smaller interval between the points at which the two partial light beams are taken out, in other words, the sensing points, in relation to the diameter of the aperture opening thereof. On the other hand, the use of the latter or small aperture lens leads to a relatively larger interval between the sensing points in relation to the diameter of aperture opening thereof. Therefore, in the case of the former lenses, the extent to which the relative position of the two images varies as the degree of focusing of the objective lens to the object is made relatively small so that it is difficult to perform focus detection with high accuracy. On the other hand, in the case of the latter lenses, the mutilation of the light beams is made very sensible, and it becomes difficult to obtain a good image quality for focus detection. Therefore, the reliability of focus detection will be largely reduced.